Previous studies have suggested that disruptions or deficiencies in folate metabolism increases, and that folate supplementation may reduce, the risk of cardiovascular disease, neural tube defects (NTDs), and cancer. Much of this effect is associated with the homocysteine (Hcy) lowering ability of folate supplementation. In recent years, the contribution of several enzymes involved in folate metabolism, essential to folate-mediated disease states, have begun to be recognized. A key enzyme is MTHFR, which catalyzes the reduction of 5,10-methylenetetrahyrdrofolate. Current understanding of effects of enzyme variants on the in vivo behavior of folate metabolism is still elementary. A first step in understanding the pharmacogenomics of folate/Hcy metabolism is to quantify the in vivo kinetics of folate pools in the context of MTHFR and other genetic variants. Testing system function is best accomplished using in vivo tracer methodologies that have sufficient analytical precision to detect differential metabolism under normal physiological conditions. 14C-labeled substrates coupled with Accelerator Mass Spectrometry (AMS) detection have the sensitivity and specificity to reveal differential metabolism at true tracer (nearly massless) doses that do not disturb or saturate normal enzymatic processes. Our long-range goal, is to establish the basis for the individual response (metabolic phenotyping) to folic acid in the context of genetic variants under normal and pathological conditions. As our next objective in pursuit of this goal, we propose to determine the in vivo kinetic of folic acid in individuals with respect to two variants of MTHFR (C677T & A1298C), MS A2756G, and methionine synthase reductase (MTRR) A66G. Our central hypothesis is that homozygotes for either MTHFR 677 and 1298 (or compound heterzygotes), MS and MTRR (or compound heterozygotes) display altered kinetic behavior and hence altered distribution of folate pools and forms available for normal metabolism. To accomplish the objectives of this application, we will pursue three specific aims: we will screen and stratify about 400 normal premenopausal women and men for MTHFR and MS and MTRR genotypes. We will determine the in vivo kinetic behavior of a tracer dose of folate in blood, urine, and stool. We will then compare the behavior of folate metabolism (functional endpoint) among the various genotypes.